Carburetor for IC engines and an idling insert therefor

ABSTRACT

A carburetor with an idling system is designed so that the full pressure differential or gradient available between approximately ambient pressure and the vacuum in the intake tube is employed for producing a critical pressure ratio of a supersonic flow in a laval nozzle. To make this possible, a fuel air emulsion formed with primary air is introduced from a mixing duct via a constricted orifice of a tubular nozzle at a bore constriction, at which there is always a sonic velocity when there is a critical and supercritical pressure ratio, into the secondary air flow where it is superfinely atomized in the secondary air flow, with a maximum velocity differential, aided by subsequent pressure surges. At least at a point far into the partial load range of operation, the idling system produces a homogeneous mixture which is homogeneously distributed in the intake tube with a practically molecular state of division so that it is even supplied to all cylinders of the engine and completely combusted with a minimum production of contaminants.

BACKGROUND OF THE INVENTION

The present invention relates to a carburetor for IC engines. Morespecifically, the invention relates to a carburetor comprising an intakeduct opening at one end into the atmosphere and connected at the otherend with an intake pipe of a manifold of an IC engine. A throttle valveis located in the intake duct so as to essentially completely shut offsame in an idling position, and an idling duct bypasses the throttlevalve. The idling duct is formed to supply combustion air for theformation of a desired.fuel-air mixture, the fuel being caused to flowby vacuum of the combustion air at a fuel outlet from a fuel duct. Theidling duct is formed with a bore constriction upstream from the outletorifice of the idling duct in the intake duct for the production of asupersonic flow.

The invention also relates to an idling insert for such a carburetorwhich has a housing with a housing body for supporting a connector forfuel and a connector for combustion air. The insert comprises an innerfuel tube connected flow-wise with the fuel connector and an externaljet tube connected flow-wise with the connector for combustion air. Thejet tube extends, concentrically around the fuel tube with the formationof a support section for support in the carburetor housing, in adirection away from the housing body.

A carburetor of this type has been proposed in German PatentSpecification No. 2,452,342. In this known carburetor, an idling ductwas located within the material of the carburetor housing and primaryair was supplied to the fuel in a vertically placed fuel duct through abranch duct arranged at an acute angle for the formation of an emulsion.At the lower end of the fuel duct the emulsion first passed into aplenum for transfer holes, which opened into the intake duct at theposition at which the edge of the throttle valve made contact in itsclosed position. At the side of the plenum opposite the inlet, the fuelduct ran into a further plenum of the idling system, from which the fuelpassed through a throttle duct, able to be set by means of a set screwand a projection on the throttle to change its cross section, to amixing chamber for the addition and mixing of combustion air. On theside of the mixing chamber opposite to the throttle hole the fuel-airmixture passes into a small tube extending under the throttle valve along distance into the intake duct and on its side adjacent to themixing chamber it has a stepped choke structure which defines a boreconstriction for producing a sonic velocity in the flow in the idlingduct. The combustion air is supplied to the mixing chamber from anintake port in the wall of the intake duct over the throttle valve via achoke, which is responsible for the degree of vacuum requisite forcausing flow of the fuel from the fuel duct.

The degree of vacuum produced in this way in the mixing chamber isconsiderable, since it has to cause the flow of the fuel out of theadjacent choke port at a velocity which, even initially, was relativelyhigh, and such fuel has to be supplied on the other side of the mixingchamber to the narrow inlet of the choke structure without the mixingchamber being fouled by carbonizing condensate on its wall faces. Thishas to be achieved despite considerable pressure losses in the plenumchamber for the transfer holes and in the plenum chamber for the idlingsystem with the throttle projection. Consequently, it is necessary forthe degree of vacuum of the combustion air in the mixing chamber to bequite high.

On the other hand, however, the pressure in the mixing chamber alwayshas to be just twice the pressure in the induction duct if a sonicvelocity in the bore constriction is desired. Therefore, if one assumesa pressure in the mixing chamber of 0.75 bar, at which the flow of theemulsion caused is just adequate, the necessary pressure in the intakeduct will be only approximately 0.4 bar at the most in order to attain asonic velocity at the constriction with the ensuing relatively fineatomization desired. Although it may be possible under ideal conditionsto construct this carburetor to attain this result during idling, thiscan be done only under the proviso that the degree of closing of thethrottle valve is high and is not impaired by inaccuracies ofmanufacture or other factors, and under the further proviso that therated idling speed of the engine is in fact attained; the idling speedmay fall to a marked extent on switching on accessories requiring powersuch as an air conditioning system, a servosystem acting against anabutment or similar equipment and when this occurs the ideal conditionswould no longer be fulfilled. A particular reason for such idealconditions not being complied with is that, when changing over to apartial load phase of operation, the throttle valve is opened only afraction so that the vacuum in the induction duct falls to some extentand the critical pressure ratio requisite for attaining sonic flow is nolonger able to be reached.

The necessary consequence of this is that, even in the lower partialload range, desired fine atomization is no longer possible and, evenduring idling pure and simple, the set condition may very easily be lostso that even the automatic switching on of the air conditioning systemmay cause the engine to stop. However, in addition, even under idealconditions which are not able to be permanently adhered to in practiceat any rate for any length of time, there is only an incomplete amountof carburetion (in the sense of reducing the fuel droplet diameter downto an almost molecular order of size), because the combustion air existsat a low pressure and moves with a low velocity, and is combined in themixing chamber with the emulsion (which also enters the mixing chamberas well with a relatively low velocity), such that, at the point ofmixing, there will be no substantial effect to decrease the dropletdiameter. Accordingly, the fuel-air mixture passes with a relativelylarge droplet diameter into the flow which is intended to be sonic, andit is only later that a reduction in the droplet diameter by the actionof pressure waves is possible. Even if sonic flow is attained in thebore constriction, there will only be a limited degree of subsequentbreaking down of the droplets in the mixture and if the sonic flowvelocity is not reached, there will be a more or less complete absenceof such breaking down of the droplet size.

From the original papers of German Pat. No. 2053991 issued to presentinventor, it is also known to have an idling system using thetransission between subsonic and supersonic flow to produce a vacuum forstimulating flow of fuel and air, and for intimate mixing anddistribution thereof.

SUMMARY OF THE INVENTION

According to the teachings of the present invention, it is desirable todevise a carburetor of the type initially specified herein whose idlingsystem assures stable running of the engine both during idling and alsounder partial load conditions and furthermore assures optimumpreparation of the mixture and a homogeneous supply thereof to allcylinders.

In order to achieve this goal, at least the end of a fuel duct is in theform of a tubular nozzle placed in a concentric supply duct for thecombustion air and the opening of the nozzle is placed at the boreconstriction. Owing to the tubular nozzle form and the concentricarrangement of the fuel duct end in a combustion air feed duct, there isa concentric flow of combustion air around the fuel duct in the samedirection as the fuel in this duct and also around the outlet orifice ofsuch duct. This combustion air simultaneously serves for cooling thefuel and for avoiding the formation of vapor bubbles in the fuel whichwould make it non-homogeneous. Owing to the fact that the orifice of thetubular nozzle is arranged at the constriction in the combustion airduct the fuel is introduced into the combustion air flowing with a sonicvelocity and is so broken down into superfine droplets, even during theprocess of mixing; furthermore pressure surges downstream from the pointof admission and mixing lead to a further intensification of the mixingeffect and of the homogenization of the mixture, as well as to a furtherreduction in the size of any large droplets still lingering on in themixture. The overall effect is therefore that there is a more or lessfull and true physical gasification of the fuel in the mixture so thatit is present therein with an almost molecular state of division.

The sonic velocity is produced at the constriction of the cross sectionwith a high degree of certainty, more especially in the partial loadrange of operation as well, since, for attaining the critical pressureratio, the entire pressure differential between more or less ambientpressure and the pressure in the intake duct is available, and, even inthe case of a pressure increase in the intake duct to over 0.5 bar, itis still possible to ensure sonic velocity at the constriction in crosssection. Even on a possible change over from a laval flow to a venturione, under certain conditions of operation there will still be a veryfine atomization and homogeneous mixing effect since only the sonicpressure surges will disappear, while the addition and mixing will stilltake place in such a way as to profit from the maximum velocitydifferential possible in this case.

Since, even with a high degree of vacuum in the intake duct at the boreconstriction there will only be a sonic velocity (and not the supersonicvelocity occurring only after flow through the bore constriction), therewill be a very stable and constant mode of operation, for all pressureratios above the critical ratio, with respect to the pumping of the fuelfrom the fuel duct with an automatically constant metering effect. Onslowing down the engine, and when there is an extremely high vacuumdownstream from the shut throttle valve (irrespective of the possibilityof sudden turning off of the fuel supply), the amount of fuel will thusnot be proportionately greater, this possibility being equally excludedeven if the engine does not idle at a constant speed. Conversely, thisstable manner of operation will also be adhered to in the partial loadrange providing that the pressure ratio does not go below the criticalvalue owing to a pressure increase in the intake duct. In the case ofany likely drop to a value below the critical pressure ratio and achange over to venturi flow, that is to say, for example on accelerationin the upper partial load range, there will admittedly be a change inthe conditions of atomization, but all the same atomization will beoptimum; and in this connection under such operating conditions noparticular importance will be attached to a particularly homongeneouspreparation of the mixture in the idling system.

Owing to the homongeneous and finely divided preparation of the mixturethere is a correspondingly complete combustion process with a reducedemission of contaminants. Accordingly maximum engine power for a givenfuel supply rate will be achieved with a minimized output ofcontaminants into the air.

European Patent No. 0 036 524 proposes a carburetion system forproducing a sonic velocity in the narrowest cross section of a lavalnozzle so as to ensure constant induction conditions in different loadranges under such carburetion conditions. The intention was, however,certainly to produce an air-rich emulsion upstream from the laval nozzleso that the emulsion would be aspirated as such without any addition ofcombustion air through the laval nozzle. If the sonic velocity is notattained this would then lead to a transfer of the fuel from the outletorifice and to a corresponding formation of condensate. In the event ofthe sonic velicity being reached, there would be no remixing of thecombustion air moving at sonic velocity with the emulsion introducedinto such flow and a substantially worse preparation of the mixturewould be probable than in the case of the carburetion system disclosedin the German Patent Specification No. 2,452,342 which is taken as astarting point for the carburetor disclosed herein.

In accordance with a preferred feature of the carburetor of the presentinvention means, such as ports, are provided for the introduction ofprimary air (for example upstream from the point of introduction of thefuel) into the fuel for emulsion formation. As a result, the carburetionsystem of the present invention entails a larger mass flow rate throughthe tubular nozzle, for conveying a given amount of fuel, than is thecase when only fuel is caused to flow therethrough. In this case, it ispossible to avoid having superfine nozzle orifices, which are difficultto manufacture, and at the same time the danger of fouling the tubularnozzle is minimized.

When, in accordance with a further feature of the carburetion system ofthe present invention, the fuel duct is in the form of a fuel tubeplaced bodily in the combustion air flow, there is not only acorresponding intensification of cooling by the surrounding air flow butfurthermore the possibility of drawing in the primary air through atleast one circumferential port in the wall of the duct for forming theemulsion. The arrangement and form of the ports may then be fully inaccord with the desired primary air rate and distribution. The primaryair supplied to the fuel also serves to cause further cooling of thefuel from the inside. The intensive cooling so ensured not onlyminimizes the danger of vapor bubble formation but at the same timeincreases the thermal efficiency. Since emulsion is not formed upstreamof the fuel tube (which so serves as a mixing tube) it is moreeffectively possible to avoid separation of the air and fuel ascomponents of the mixture than would be the case if the primary air wereto be introduced at a point far upstream from the supply of the fuel orof the emulsion into the combustion air.

In keeping with a further feature of the carburetor of the presentinvention, there is a terminal bore constriction (more especially to abore cross sectional area between 0.03 and 0.3 sq mm or preferably toabout 0.12 sq mm) in the fuel duct for the emulsion with a relativelysmall, but not extremely small, size. As a result, in view of thepresent strong vacuum, there will be an effective control of the desiredmetering of the fuel without any excess thereof.

Since no throttling of the supply of fuel is aimed at upstream from thefuel duct (such throttling would otherwise cause unnecessary losses inthe flow), it may be, in order to draw in a defined and desired amountof primary air, an advantage in addition to suitably dimensioning theports in the wall of the duct to have a pre-choke upstream from suchports in order to ensure a suitable degree of vacuum in the fuel duct.This pre-choke may be in the form of a constriction of the fuel duct toa reduced bore area (more specifically to an area of 0.03 sq mm to 0.3sq mm and preferably 0.12 sq mm) in tune with the desired pressure andflow conditions sought. As a rule, the optimum cross sectional area willbe the same as that of the constriction in the tubular nozzle, it havinghowever to be taken into account that in the assumed preferred case thenozzle is for emulsion while the pre-choke constriction is for fuelalone.

The port or ports in the fuel tube are intended, under the pressureconditions which become established, to meter in a certain amount of airto mix with the fuel and form the emulsion, and may then preferably havea total cross section of between 0.1 sq mm and 1.0 sq mm or, moreespecially, approximately 0.45 sq mm. In place of a single large port itis more expedient to have a plurality of small ports, which are morereadily produced with the desired cross sectional area as part of theprocess of manufacture, and which prevent any unintentional discharge offuel under transient conditions, more especially if they are on the topside of the fuel tube.

The preferred dimensions of the separate bore constrictions of 0.03 sqmm to 0.3 sq mm (preferably 0.12 sq mm) for the tubular nozzle, of 4 sqmm to 40 sq mm (preferably approximately 16 sq mm) for the supply ductfor combustion air, of 0.03 sq mm to 0.3 sq mm (preferably approximately0.12 sq mm) for the pre-choke and of between 0.1 sq mm and 1.0 sq mm(preferably approximately 0.45 sq mm) for the port for the entry ofprimary air, result in optimum running conditions for a 2.8 literengine. For engines with other cubic capacities the optimum valueswithin the ranges set forth above will be larger or smaller, althoughthe relationship between the dimensions will be substantially the same.

Despite the absence of air ports in the fuel line in view of theintroduction of primary air only at the outlet end of the idling duct,in order to safely prevent the drawing in of fuel from the float chamberduring pauses in operation, it is possible to have a valve for shuttingoff the fuel line (more specifically at a point upstream from the fueltube) automatically. If this valve is placed as close as possible to theoutlet of the idling duct, this will also serve to minimize the amountof fuel which necessarily drips out of the fuel duct when there is apause in operation.

In keeping with a further feature of the carburetor of the presentinvention, it is expedient to have the air duct and/or the fuel duct ofthe idling duct in the form of tubes standing free of the carburetorhousing. Apart from the reduction in design and manufacturing costsbeing less than for carburetors having the ducts within the carburetorhousing, this means that with regard to the air duct, there will be arespective freedom with regard to the design of the upstream connectionof the air line which does not have to be directly connected with theair filter, but may also be in the form of a branch from the cylinderhead air vent duct so that even at a point upstream from the air filterit is possible for the oil mist in the filter to be drawn off, possiblywith the addition of air from the air filter which is drawn through thesection, leading to the air filter, of the cylinder head venting duct.

With regards to the fuel duct, the design in the form of a duct standingout from the side of the carburetor housing gives the special advantageof cooling the hot fuel drawn in from the float housing. This is moreespecially of value in countering hot start problems.

In order to prevent an excessive amount of heat being transferred fromthe hot wall of the carburetor to the downstream end of the fuel line,something that would be prone to cause irregular operation owing to theformation of bubbles of vapor in the fuel line, the latter duct may endin a connector supported in a connecting part of material with a lowthermal conductivity, more especially plastic. This avoids the directtransfer of heat from the hot metal parts.

As part of a further feature of the carburetor of the present invention,the fuel line is arranged to terminate in the connector with its axisplaced transversely in relation to the axis of the fuel duct, thismaking it possible to save space here as is usually desired.

In accordance with still a further feature of the carburetor of thepresent invention, the arrangement serves to form an annular trapchamber for any small vapor bubbles tending to move back out of the fuelpipe and likely to coalesce as large vapor bubbles causing irregularrunning if they are allowed to get into the fuel line. The connectorextends downwards through the trap chamber to form its inner wall face.The inlet port of the fuel duct is placed to the side above the level ofthe outlet orifice of the connector. Any small vapor bubbles tending tomove out of the fuel tube are thus caught in the trap chamber andstopped from transferring into the lower outlet orifice of the connectorof the fuel line so that there is no interruption in the continuoussupply of the fuel therethrough. The vapor bubbles whose size are in anycase limited to the volume of the trap chamber, may be drawn again intothe fuel duct during the course of further flow of the fuel and mayemerge with the fuel or the emulsion thereof without causing anyirregularity, from the fuel duct into the combustion air flow.

In accordance with an even further feature of the carburetor of thepresent invention, the arrangement is such that the cross section of theflow of fuel from the flow chamber to the inlet port of the fuel tube,that is to say the cross section of the fuel line, of the connectorthereof, of its outlet orifice, of the trap chamber and of the point oftransfer from the outlet orifice to the trap chamber, is at leastapproximately constant. This results in an even flow velocity and aninsensitivity with respect to transients such as vibrations, differentslopes on traveling up and down hill, the formation of dead zones andthe like.

For not only simplifying production, but more especially to makepossible fitting to existing systems, it is possible for the outletpart, having the fuel duct of the idling duct, to be in the form of aseparate housing, which has a nozzle tube (forming the combustion airduct) adapted to be fitted to extend through the wall of the carburetorhousing and/or of the intake duct, respectively, as for example at aposition adjacent to a base plate of the carburetor.

Adaptation to suit the respective operating conditions may be madepossible by providing adjustable means for securing the precise positionof the port of the fuel duct in relation to the cross section boreconstriction in the combustion air duct so that, if required, fineadjustment may be undertaken on any IC engine without, normally, anylater adjustment being needed.

In order to ensure a particularly low loss inlet flow and efficientacceleration into the supersonic range before detachment and flowtransition take place, the bore constriction in the combustion air ductmay be in the form of a converging-diverging laval nozzle.

As a further feature of the carburetor of the present invention, theaxis of the outlet part of the idling duct may be at an angle of 0° to30° and more especially at an angle of at least about 10° to the centeraxis of the intake duct.

Furthermore, the axis of the outlet part of the idling duct may be at anangle (as seen in a horizontal plane) of 15° to 40° and more especiallyof about 20°. to a line drawn radially from the center axis of theintake duct, such angle being measured at the point of intersection ofthe axis of the outlet part of the said idling duct with theprolongation of the outer face of the intake duct.

With such slopes of the axis of the outlet part of the idling duct in adownward and in a lateral direction towards a more tangential flow it ispossible to minimize the vacuum at the outlet orifice of the idling ductand to optimize the mixing effect, more particularly in the partial loadrange. The introduction of the mixture from the idling system, with ahigh velocity but with a limited mass flow rate therefore, has thetendency to keep the mixture flowing in a helical flow path runningdownwards in the intake duct or pipe, respectively, this favoring aprogressively complete mixing with the mixture flowing past the throttlevalve with minimum flow losses. While the downward slope more especiallymakes a contribution to minimizing the flow losses and, in the partialload range, to increasing the local vacuum at the outlet orifice of theidling duct the inclination to the side is more especially helpful withregard to improving the mixing effect; since at this point in time thefuel is already in a practically "carbureted" or gasified form, there islittle risk of centrifugal separation of fuel droplets and the formationof condensate, more especially since the flow at the same time entersthe substantially widened intake tube owing to the downward motion.

In accordance with still another feature of the carburetor of thepresent invention, a housing is provided having a housing body forsupporting a connector for fuel and a connector for combustion air aswell as an inner fuel tube connected with the fuel connector for flowtherebetween, and an outer nozzle tube connected with the combustion airconnector for flow therebetween, the nozzle tube extending away from thehousing body concentrically around the fuel tube to form a supportportion for the support of the carburetor housing.

This feature of the carburetor of the present invention constitutes anidling insert fitting which is a separate and compact part producedseparately from the carburetor so that it may be sold for modificationof a pre-existing carburetor. This possibility of production anddistribution of such separate components has a special degree ofimportance as part of the present invention, since it makes possible theuse of the teachings of the invention independently from mass producedarticles coming from automobile and carburetor manufacturers so that theinvention may be put into practice at the option of the consumer or carowner and he or she may make his own contribution to economizing in theuse of energy and protecting the environment.

Furthere details, features and beneficial effects of the carburetor ofthe present invention will be apparent from the following description ofone working embodiment thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic and simplified generally vertical sectionalview of a carburetor constructed according to the teachings of thepresent invention.

FIG. 2 is an enlarged longitudinal section through the outlet part ofthe idling duct of the carburetor shown in FIG. 1.

FIG. 3 is a top plan view, partially in section, of the outlet part ofthe idling duct shown in FIG. 2 viewing same from above.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A carburetor is shown in FIG. 1 with an air filter 1, a carburetorhousing 2 and an intake duct 3 passing through the housing 2 for theaspiration of outside air through the air filter 1. The carburetorhousing 2 includes a base plate 4 for connection with an intake pipe 5of an intake manifold 6, which supplies the cylinders of an IC engine ina conventional manner with fuel-air mixture and on which the base plate4 is mounted using a conventional gasket 7.

A butterfly throttle valve 8 is located in the intake duct 3 so as topractically fully close the intake duct 3 in the idling setting.

The carburetor shown is in the form of a governor carburetor and has anintake duct 9 of a second stage, whose throttle valve 10 starts to openwhen there is a substantial increase in the engine speed.

In a conventional manner, the carburetor housing 2 is produced as acasting and in addition to the base plate 4 has two superposed housingparts 11 and 12, the section in FIG. 1 being such as to run along theaxes of the intake ducts 3 and 9 in the base plate 4, while on the otherhand adjacent to the housing parts 11 and 12 it runs through a plane infront of it extending through a float chamber 14.

Under the control of a float 13, fuel moves into the float chamber 14,whence the fuel is abstracted via a fuel line 15 in the form of a stiffor flexible pipe 15 standing free from the side of the carburetorhousing 2. Oil mist, as produced in the crank case and the entire engineblock, is supplied through a cylinder head vent line 16 to the airfilter 1. In the illustrated embodiment, the cylinder head vent line 16does not run directly to the air filter 1 but into an air line 17connected with the air filter 1. The fuel line 16 and the air line 17form parts of an idling duct structure generally identified withreference numeral 18, with which the fuel and the air may be supplied toan idling system opening into the intake duct 3 downstream from thethrottle valve 8.

The other features of the carburetor, as for example an acceleratingpump, may be of a conventional design and are therefore not in need ofexplanation here. It does, however, remain to be stressed that thethrottle valve 8 of the carburetor of the present invention has to standin a setting in which it practically completely shuts off the intakeduct 3 of a first stage so that essentially no air flow is possible pastthe edge of or through the throttle valve 8 and there are no ducts, orat any rate no open ducts, which would lead to leakage of air. At theedges of the throttle valve 8 in its idling setting it is posssible tohave conventional transfer ports 19, if there is no other transfersystem, for the supply of mixture in the transitional stage of operationbetween idling and partial load.

The outlet part 20 of the idling duct structure 18 is shown in moredetail in FIGS. 2 and 3. As shown in these Figures, the fuel line 15comes to an end in a connector 21 and the air line 17 ends in aconnector 22, the connectors 21 and 22 being supported in a housing 23,which essentially consists of a jet nozzle tube 24 for the formation ofa supply duct 25 for the combustion air around a fuel tube 26, whichforms a fuel duct 27. Furthermore, adjoining a supporting portion 28,formed essentially by the nozzle tube 24 to fit into the carburetor wall2, the housing 23 comprises a rear housing body 29 adjacent to theconnectors 21 and 22 with end faces 30 next to the supporting portion 28and a connection part 31 of material with a low thermal conductivity asfor example plastic in the instant case, whereas all other parts aremade of metal.

At its front end, the fuel tube 26 has a tubular nozzle 32 with a boreconstriction 33 having a cross section of, for instance, 0.12 sq mm, thesame also forming a port 34 for the emergence of fuel or emulsion, asthe case may be. In its rear top part the fuel tube 26 has round,axially spaced, ports 35 which, in the present example, are two innumber and have a diameter of 0.5 mm and 0.6 mm, respectively, that isto say with a sum cross section of approximately 0.45 sq mm. Such portsmake possible flow of the air from the flow around the fuel tube 26 intothe fuel duct 27 so that a fuel emulsion is formed therein. A pre-choke36 is located upstream from the ports 35 and in the present example itis in the form of bore constriction 37 with a cross sectional area of0.12 sq mm.

The fuel duct 27 opens, by way of an inlet port 38, into a trap chamber39 through which the connector 21 of the fuel line 15 extends and whichis machined in the connection part 31 of plastic. An outlet port 40 ofthe connector 21 is in this case lower than the lower edge of the inletport 38 of the fuel duct 27 and therefore also at a lower level than thetrap chamber 39 so that on the supply of fuel from the outlet port 40 ofthe connector 20 via the trap chamber 39 into the inlet port 38 of thefuel duct 27 there is a sort of inverted syphoning effect.

The tubular nozzle 32 with the bore constriction 33 of the fuel tube 26is placed in a constriction 41 upstream from an outlet port identifiedby reference numeral 42, of the idling duct structure 18 into the intakeduct 3. The constriction 41 is in this respect in the form of a sort ofconvergent-divergent laval nozzle so that if a critical pressuredifferential or ratio between the planes A and B is exceeded in theconstriction 41, there will be a flow with a sonic velocity and in thefollowing somewhat diverging part of the nozzle tube 24 there will be asupersonic velocity, until detachment and flow transition occur. In thecase of a supercritical pressure differential this will be, at thelatest, in the plane B, that is to say in the plane of the outlet port42. In the present example, the bore constriction 41 has an area ofcross section of approximately 16 sq mm, this being believed to be thesize for optimum operation of a 2.8 liter engine.

The fuel tube 26 and the nozzle tube 24 are placed concentrically aboutan axis 43 that intersects the axis 44 (which is perpendicular to it) ofthe connector 21 of the fuel line 15. Furthermore the axis 45 of theconnector 22 of the air line 17 is perpendicular to the axis 43 but doesnot have to intersect with it.

As shown in FIG. 3, the connection part 31 together with the fuel tube26 is swivel mounted in the housing body 29, with corresponding turningof the connector 21 being provided for since the connector 21 runs in aslot 46 in the housing body 29. The axis 47 of the slot 46 is notperpendicular but is at an angle to the axis 43 so that the swivelmovement of the connection part 31 and of the fuel tube 26 with arocking of the connector 21 also leads to an axial motion of the fueltube 26. This makes possible accurate adjustment of the position of theport 34 of the tubular nozzle 32 in relation to the constriction 41 inaccordance with specific requirements. In the present case, the lengthof the slot 46 is intended to allow a twist of the connection part 31 of30° and is set at an angle of 13° obliquely in relation to the axis 43so that the amount of adjustment is of the order of 1 mm.

For assembly in the position shown in FIG. 1, it is possible for theentire nozzle tube 24 to be inserted into a suitable hole in thecarburetor housing 2 until it abuts the front end faces 30 of thehousing body 29. As already indicated, in connection with thedescription of FIG. 1, the axis 43 may be inclined at an angle to thehorizontal, the angle having a possible range of approximately 0° to 30°and in the present case it may have a value of 10° owing to designlimitations occasioned by the overall height of the base plate 4. In amanner which is similar, but which is not illustrated, the axis 43 doesnot have to intersect with the center axis of the intake duct 3 and itis possible for there to be an oblique setting of the axis 43 clear ofthe radial setting such that the emergency of the flow from the outletport 42 is more tangentially directed into the interior of the intaketube 3. Such an angle to the radial direction may be between 15° and 40°and, in the present case, may be taken to be 20° as measured at thepoint of interference, generally identified by reference numeral 48 inFIG. 1, of the axis 43 with the extension of the outer face of theintake duct 3.

During idling the throttle valve 8 is closed so that the vacuum producedin the intake duct 3 downstream from the throttle valve 8, owing to theintake strokes of the pistons, acts in full on the outlet port 42 andthrough the latter in the idling duct structure 18. The result is thatair is firstly drawn in through the air duct 17 and the oil mist presentin the cylinder head vent line 16 will be entrained as well, such mistbeing supplemented by air from the air filter 1. This air current willonly undergo a small drop in pressure so that the pressure in the planeA will be more or less atmospheric and at the intake duct 3 adjacent tothe outlet port 42 there will be, for example, a pressure of only 0.4bar. This means that the critical pressure ration between the planes Aand B has been substantially exceeded so that a sonic flow willestablish itself in the plane of the constriction 41 and will befollowed by a supersonic flow.

Owing to the marked pressure drop in the inlet part of the boreconstriction 41 and the change over from static pressure into dynamicpressure of the air flow, there will be a correspondingly intensesuction effect on the fuel thereat through the port 34 of the tubularnozzle 32 and fuel therefore will be supplied through the constriction33 to the air flow at a metered rate. At the same time, however, primaryair will be drawn from the air flow around the connector 22 through theports 35 at a point upstream from the tubular nozzle 32 and introducedinto the fuel tube 26 where it will form a fuel air emulsion with thefuel in the fuel tube. Thus, at the port 34, the fuel in the form ofsuch an emulsion will pass into the combustion air flowing in the supplyduct 25, such entry being at a position at which there is an extremelylarge velocity differential owing to the sonic velocity of thecombustion air. As a result, the fuel, emerging with a very much lowervelocity, will be broken down into very small droplets and atomized sothat downstream from the constriction 41 there will be a fuel-airmixture with the desired lambda value having a very homogeneousdistribution, at least at the outlet port 42. At the latest, at theoutlet port 42 there will then be a further disintergrating effect onany large droplets still present owing to the pressure surge when thereis a flow transition to an ultrasonic value. In the manner indicated inFIG. 1, a flow emerges downwards and sideways from the outlet port 42and passes into the intake tube 3. It flows turbulently through the tube3 and fills it very rapidly and homogeneously with finely divided fuelin particles with a more or less molecular order of size.

This condition remains unchanged as long as the critical orsupercritical pressure differential is maintained between the planes Aand B, in which respect even a highly supercritical pressuredifferential or ratio hardly causes any change in the atomization stateat the constriction 41, since the velocity is always supersonic at thisposition. In the event of the pressure differential being subcriticalunder full load or in transient conditions, as for instance duringacceleration, the part of the nozzle tube 24 between the planes A and Bwill function as a venturi tube, in which respect, however, the supplyof the fuel will be at the point of maximum velocity differentialbetween the combustion air flow and the fuel so that, in theseconditions as well, optimum atomization still takes place, although itis of only slight importance under such load conditions. It is, however,important that in steady state conditions, a critical pressure ratioexist far into the partial load range so that the ideling mixture willbe supplied under constant, stable conditions. Furthermore, the oil mistfrom the cylinder head vent duct 16 is supplied to such idling mixturein the way indicated directly, or via the air filter 1, so that the mistis dealt with in a manner conducive to economy in energy and toprotection of the environment.

Since the fuel is supplied via the fuel line 15 without any notablepressure losses, it may be expedient to step up the degree of vacuum inthe fuel duct 27 at the port 35 in order to guarantee the requisiteinput of primary air. This is made possible by the pre-choke 36, thecross sectional area of the constriction 37 thereof being adapted, onthe one hand, to the desired pressure drop and, on the other hand, tothe overall pressure drop as far as the port 34, in order to attain adesired exit velocity for the emulsion. Typically, the size of the areaof the constriction 37 will be, dependent on the engine cubic capacity,between 0.03 sq mm and 0.3 sq mm. In view of the selected cross sectionsize of 0.12 sq mm of the constriction 33 through which the emulsionflows, in the present example, a cross section size of 0.12 sq mm can beselected for the constriction 37 having fuel alone flowing through it.In the case of the selected summated cross section of the ports 35 ofapproximately 0.45 sq mm there will be an optimum formation andpropulsion of the emulsion through the tubular nozzle 23 under theaction of the combustion air, which always flows through theconstriction 41 with a sonic velocity. A size of the cross section atthe constriction 41 of approximately 16 sq mm then leads to a supply ofcombustion air to the flowing fuel at such level as to ensure a properlyignitable mixture and at such a rate that, in the case of a 2.8 literengine the idling speed, will be 600 to 700 rp.

The choking constrictions 33 and 37 are not able to prevent fuelsyphoning from the float chamber 14 of its own accord if the enginestops, since access of air into the fuel line 15 is not possibleupstream from the connector 21. For this reason the fuel line 15 isprovided with a valve 49 which automatically shuts off the fuel line 15below a head, for instance, of 4 cm of gasoline in the line. Therefore,at the most, only dribbling of fuel downstream from the valve 49 will bepossible. The volume of such fuel may be minimized and, owing to thecomplete shutting off at the level of the valve 39, it will only be ableto flow (if at all) slowly; it is in this way that the amount of fuelleaking, in the case of the illustrated form of the carburetor of thepresent invention, may be limited to the content of the fuel tube 26downstream from the ports 35.

The connection part 31 made of material with a low thermal conductivityprevents any substantial transfer of heat between the hot peripheralwall of the housing body 29 and the connector 21 and also the fuel tube26, it being significant in this respect that the connector 21 is fittedin the slot 46 with some lateral play. This means that the cooling ofthe fuel tube 25 by the surrounding combustion air flow in the supplyduct 26, and also by the primary air drawn in through the ports 35,still will be effective, even in the rear part of the fuel tube 26, sothat the same will be relatively cool even at the inlet port 38.

Transfer of any vapor bubbles, nevertheless formed in the fuel tube 26,into the fuel line 15 is prevented by the trap chamber 39, since vaporbubbles tending to move back towards the fuel line 15 will be retainedat the uper wall of the trap chamber 39 until they are moved (perhapsafter increasing somewhat in size and bulging to a greater extent downinto the fuel space) back into the fuel tube 26 and leave it togetherwith the fuel or the emulsion (as the case may be) via the port 34,something that does not give rise to any irregularities in operation.Owing to the fact that the cross section area of the fuel line 15, ofthe connector 21, of the annular trap chamber 39 and of the transitionbetween the outlet port 40 and the trap chamber 39 have been designed tobe generally equal in size, there will be a regular flow of the fuelbetween the float chamber 14 and the inlet port 38 of the fuel tube 26and such flow will be unlikely to be disturbed, and more especially inthe case of a relatively high flow velocity through a small crosssection, will make a substantial contribution to avoiding the formationof vapor bubbles, even under very unfavorable conditions.

The working example of the carburetor of the present invention describedabove leads to the advantages described initially herein; a moresignificant point, in this respect, is that the relatively high pressureat the plane A makes it possible for the critical pressure ratio to bemaintained far into the partial load range so that consequently moreregular operation of the idling system may be ensured. Since,furthermore, in the partial load range as well, a corresponding flow ismaintained through the idling system, and such flow may certainconstitute a substantial part of the fuel-air mixture made available forthe cylinders, the optimum operation, at any rate, of this part leads toa significant increase in mileage and a drop in contaminant emission inthe partial load ranges as well.

For achieving a maximum vacuum in the idling setting the throttle valve8 can be fully closed in this position--with the possible exception ofsmall gaps caused by manufacturing tolerances. This position of thethrottle valve 8 in the idling setting does also form the basis for theindicated metering of the ducts of the idling system.

However, a certain problem might arise if the transfer port 19 which isusually provided as concentrical elongated slot is also shut offcompletely in this position by the edge of the throttle valve 8 from thevacuum below the throttle valve 8, since then, with the transition tothe partial load range, there may occur a nonsteady phase with a fuelsupply reduced as against the desired value because of said load, i.e.,an "acceleration gap", since the flow of the transfer port 19 proceedingfrom the previous zero-flow starts with delay.

For avoiding such nonsteady states of operation it can also be providedthat the edge of the throttle valve 8 comprises a small gap with amaximum diameter of e.g. 0.2 to 0.3 mm to the wall of the intake duct 3in the idling setting, i.e. that the throttle valve 8 does notcompletely shut off the flow in the intake duct 3 but only throttles it.In such a case there is, also in the idling setting, a certain basicflow of fuel and/or emulsion from the transfer port 19 and acorresponding air supply from the intake duct 3. With an appropriatecompensation of said additional fuel and air supply by a respectivelyreduced fuel and air supply from the idling duct means 18 there are thesame operating conditions as with the above embodiment.

The carburetor of the present invention has a number of advantages, someof which have been described above, and others of which are inherent inthe invention. Also modificaitions can be made to the carburetor of thepresent invention without departing from the teachings of the presentinvention. Accordingly the scope of the present invention is only to belimited as necessitated by the accompanying claims.

I claim:
 1. A carburetor for an IC engine comprisingan aintake duct (3)opening at one end into the atmosphere and connected at the other endwith an intake pipe (5) of a manifold (6) of an IC engine, a throttlevalve (8) located in said intake duct (3) so as to at least essentiallyshut off said intake duct (3) in an idling position, and idling ductmeans (18) bypassing said throttle valve (8), said idling duct meansincluding feed duct means (24) for supplying combustion air for theformation of a desired fuel-air mixture, said means for supplyingincluding a fuel duct means and a fuel outlet orifice connected to saidfuel duct means (27), said idling duct means (18) being formed with abore constriction (41) upstream from said outlet orifice (42) of theidling duct means (18) in the intake duct (3) for the production of asupersonic flow, and characterized in that said fuel duct means (27) isin the form of tubular nozzle (32), said fuel duct means being disposedwithin said feed duct means for the combustion air, said tubular nozzlebeing located concentrically in said feed duct means (24) for thecombustion air, said tubular nozzle being connected to a tube (26) withsaid tube having at 1east one peripheral port (35) for the inlet ofcombustion air, said tubular nozzle having said outlet orifice (34)which is located in said bore constriction (41) and said outlet orificebeing located adjacent a sonic Laval plane defined by said boreconstriction, said concentric feed duct means having a downstream endrelative to said orifice (34) which includes a diverging portion forcontaining supersonic flow under Laval conditions.
 2. The carburetor asclaimed in claim 1, characterized in that the tubular nozzle (32) has aterminal bore construction (33) down to a bore cross sectional area ofbetween approximately 0.03 sq mm and 0.3 sq mm.
 3. The carburetor asclaimed in claim 1 characterized in that the bore construction (41) ofthe feed duct (25) for combustion air has a free bore with a crosssectional area between approximately 4 sq mm and 40 sq mm.
 4. Thecarburetor as claimed in claim 1, characterized by a pre-choke (36)upstream from the port (35) for ensuring a degree of vacuum in the fuelduct (27) for causing the entry of the primary air.
 5. The carburetor asclaimed in claim 4 characterized in that the pre-choke (36) is in theform of a bore construction (37) in the fuel duct (27) which has a borecross sectional area between approximately 0.03 sq mm and 0.3 sq mm. 6.The carburetor as claimed in claim 1 characterizxed in that the totalbore area of the one or more ports (35) is between approximately 0.1 sqmm and 1.0 sq mm.
 7. The carburetor as claimed in claim 1 characterizedby a valve (48) in the fuel line (15) for shutting off the fuel line(15) upstream from the fuel tube (26) when operation of the carburetoris interrupted.
 8. The carburetor as claimed in claim 1 characterized inthat the air line (17) and/or the fuel line (15) of the idling ductmeans (18) are in the form of lines standing free of the side of thecarburetor housing (2).
 9. The carburetor as claimed in claim 8characterized in that the fuel line (15) ends in a connector (21) whichis supported in a connecting part of material which has a low thermalconductivity.
 10. The carburetor as claimed in claim 9 characterized inthat said connector part is made of a plastic material.
 11. Thecarburetor as claimed in claim 9 characterized in that the fuel line(15) ends in the connector (21) so that its axis (44) is transverse inrelation to the axis (43) of the fuel duct (27), in that the outletorifice (40) of the connector (21) of the fuel line (15) is placed at alower level than the inlet port (38) of the fuel duct (27) and in that,between the outlet orifice (40) of the connector (21) and the inletports (38) of the fuel duct (27), there is an annular trap chamber (39)for vapor bubbles.
 12. The carburetor as claimed in claim 11characterized in that the cross sectional bore areas of the fuel line(15), of the associated connector (21), of its outlet orifice (40), ofthe trap chamber (39) and of the connection between the outlet orifice(40) and the trap chamber (39) are at least approximately equal in size.13. The carburetor as claimed in claim 1 characterized in that the endhaving the fuel duct (27) of the idling duct means (18) is in the formof a separate housing (23), which has a tubular nozzle (24) forming thecombustion air feed duct (25) and which extends through the housing (2)of the carburetor or of the air intake duct (3).
 14. The carburetor asclaimed in claim 1 characterized by having means for adjusting theposition of the orifice (34) of the fuel duct (27) in relation to thebore construction (41) of the feed duct (25) for the combustion air. 15.The carburetor as claimed in claim 1 characterized in that the partforming the bore construction (41) of the feed duct (25) for combustionair is in the form of a converging-diverging laval nozzle.
 16. Thecarburetor as claimed in claim 1 characterized in that the axis (43) ofthe outlet part (20) of the idling duct means (18) is inclined downwardsin relation to the center axis of the intake duct (3) at an angle ofbetween approximately 0° and 30°.
 17. The carburetor as claimed in claim16 characterized in that said angle is at least approximately 10°. 18.The carburetor as claimed in claim 1 characterized in that the axis (43)of the outlet part (housing 23) of the idling duct means (18), at thepoint of intersection (48) of the axis (43) of the outlet part (20) ofthe idling duct means (18) with the prolongation of the outer face ofthe intake duct (3), makes an angle in relation to a line radial to thecenter axis of the intake duct (3) and in a horizontal plane of betweenapproximately 15° and 40°.
 19. The carburetor as claimed in claim 18characterized in that said angle is approximately 20°.
 20. An idlinginsert for a carburetor comprising a housing (23) with a housing body(29) supporting a connector (21) for fuel and a connector (22) forcombustion air, said insert comprising an inner fuel tube (26) connectedflow-wise with the fuel connector (21) and an external jet tube (24)connected flow-wise with the connector (22) for combustion air, said jettube (24) extending concentrically around the fuel tube (26) with theformation of a support section (28) in the carburetor housing (2), in adirection away from the housing body (29).